The overall goal of the proposed research is to delineate the temporal dynamics and neural substrates of attentional processes in visual discrimination and to thereby contribute to an understanding of the functional neuroanatomy of selective attention mechanisms. Selection of particular aspects of environmental stimuli for further processing forms a key aspect of perceptual and cognitive operations and is typically called selective attention. The study of selective attention has been concerned with understanding the processes underlying these selective operations. However, we propose that selective attention can be considered to be composed of at least three interrelated classes of processes: (1) processes involved in focussing or shifting attention, (2) processes that perform the selective operations (which are the most commonly studied) and (3) processes that sustain selective attention. Accordingly the proposed study is concerned not only with the processes which underly the "consequences of attention", i.e. selective information processing, but also with the underlying mechanisms involved in setting up and maintaining selective information processing, and in differentiating these from the mechanisms involved in the selective process per se. The proposed experiments address these three classes of attentional processes using event related brain potentials (ERPS) as indices of the macroscopic neural activity that underlies them. The chief aim of this research is to delineate the temporal course and loci of operation of these processes. This can effectively be addressed by the powerful method of Brain Electric Source Analysis (BESA), that provides unique intracranial localization of multiple dipolar ERP sources, and the temporal course of activation of each spatially discrete equivalent dipole as well. The coordinates of each individual dipole, given whithin a spherical model of the head, will be mapped for each individual onto a three-dimensional representation of the brain obtained by MRI, so as to define dynamic spatiotemporal patterns of cortical activity associated with attentional aspects of discriminative processing.